Insurance on the World Trade Centre's twin towers will pay out for the loss of only one tower because experts believed that the collapse of both towers simultaneously was too far-fetched to be worth insuring, it emerged yesterday.

The Port Authority of New York, owners of the towers, are likely to receive an insurance payout of $1.5bn, far below the $5bn-plus value put on the towers before their collapse and only a fraction above their $1.2bn construction cost in the early 1970s.

A spokesman for the US Insurance Information Institute said: "The possibility of the loss of both structures was seen as so remote that cover was not taken out on those lines. The $1.5bn of coverage was purchased on the basis of a probable rather than a possible maximum loss."

Estimates of the total cost of the terror attack on the US yesterday spiralled to $30bn, amid growing fears that Lloyds of London will be crippled by huge payouts.

The losses dwarf the previous record of $16bn paid out after Hurricane Andrew in 1992 and will be the biggest insurance catastrophe in history. Lloyd's is expected to be worst hit because its aviation syndicates insure around 35% of the world's planes.

Around 2,800 wealthy private individuals still make up around 30% of the Lloyd's of London market. Despite reforms, many Lloyd's "names" still have unlimited liability which will leave them bankrupt if claims spiral.

Yesterday Lloyd's was unable to identify the underlying scale of its losses, but promised it will issue early estimates today.

Until the events of this week, insurers regarded a collision of two planes over a major city as the worst conceivable "realistic disaster scenario." But economist Tim Congdon, of Lombard Street Research, said: "Yesterday's tragedy is far worse than any conceivable 'realistic disaster scenario' considered by insurance companies. The loss will have a devastating effect on some US insurance companies ... and Lloyd's will of course also take a bad hit."

The third largest underwriter at Lloyd's, Amlin plc, yesterday postponed the publication of its half-year results in the light of the tragedy, sparking a 30% slide in its share price. Other quoted Lloyd's underwriters, including Hiscox and Brit Insurance, also fell heavily.

Munich Re, the world's biggest reinsurer, was the first to pencil in figures for the scale of losses. It put its initial estimate of liabilities at nearly $1bn and said that payouts will severely hurt profits.